In automatic transmissions for motor vehicles, such as known (for example) from DE19858541A1, the transmission ratio stages are adjusted by hydraulically actuated shift elements, hereinafter also referred to as “clutches.” Clutches may connect both two rotatable elements together, and one element connected in a torque-proof manner to the transmission housing to one rotatable element. In the latter case, one also speaks of a brake. Thereby, the shifting elements are formed as frictional-locking multi-disk clutches. In the automatic transmission disclosed in DE19858541A1, six transmission ratio stages are adjustable for forward travel, whereas, for the presentation of each transmission ratio stage, three shift elements must be closed simultaneously.
With known automatic transmissions, actuation (i.e., the closing of the shift elements for the transfer of torque) takes place hydraulically, i.e. by actuation devices in the form of piston-cylinder units, which are subjected to a pressure medium, typically transmission oil, and convert hydraulic energy into mechanical energy. Such a piston-cylinder unit is also referred to as a “clutch cylinder.” A clutch pressure chamber is formed from the piston and the cylinder; for the actuation of the shift element, this is subjected to transmission oil that is under actuating pressure or clutch pressure. Under the action of the pressure, the piston is typically pressed against a multi-disk pack counter to the force of a return spring, which makes these two components connected to each other in a torque-proof manner. If the pressurization of a shift element is referred to below, it will be understood that the pressurization of the clutch cylinder or the clutch pressure chamber that is encompassed by the shift element is provided.
The pressure oil is conveyed by a motor-driven transmission pump as a pressure source. The actuating pressure is adjusted by a pressure-adjusting device of a hydraulic transmission controller. By the hydraulic transmission controller or its pressure control devices, it is also possible to reduce the actuating pressure to ambient pressure, by which the clutch pressure chamber is pressureless and a force effect on the piston is no longer present. A pressureless clutch cylinder is also designated as “vented.”
In particular, with frictional-locking shift elements, the actuating pressure needs to be maintained at a pressure level that produces a pressing force of the multi-disks in the shift element that is sufficient for the transfer of torque during the entire actuation period.
With the aim of reducing fuel consumption, or the reduction of carbon dioxide emissions, various methods for operating a vehicle drive, comprising a drive motor and a transmission, are used. The drive motor is typically an internal combustion engine. A known measure is the so-called “start-stop operation.”
In a variant of a start-stop operation disclosed in DE19961796C1, the internal combustion engine is switched off, for example, when the vehicle is coasting and a vehicle standstill is imminent. By switching off the internal combustion engine, the transmission pump would also be stationary, such that the conveying of the operating medium or the production of pressure, as the case may be, would collapse in the entire hydraulic system. Thereby, the lubrication of the transmission components rotating while coasting would no longer be ensured, which would lead to increased wear or damages to the relevant components. An insufficient supply of lubricating oil, particularly in the bearings, may lead to damages to and ultimately the failure of the transmission. For this reason, with known automatic transmissions, an additional auxiliary pressure source is provided for such conditions, which may provide a volume flow of an operating medium under pressure, and thus take over the oil supply of the automatic transmission when the transmission pump is stationary. Such an auxiliary pressure source may be formed, for example, as an electrically driven pump in DE19961796C1. An alternative auxiliary pressure source would be a pressure accumulator, which is known from DE102006014756A1 of the applicant. The provision of an auxiliary pressure source disadvantageously means a considerable expense of costs and weight, and requires additional installation space. In addition, with the use of an electrically driven transmission pump, software-based measures for creating an emergency strategy may be necessary in the event that the electrically driven transmission pump fails.